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Molecular and Biological Mechanisms of Host Plant Responses to an Insect Vector and a Bacterial Pathogen

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Abstract

Zebra chip is an emerging plant disease of solanaceous crops that poses a threat to global food security. Understanding plant biological and molecular responses to Bactericera cockerelli and ‘Candidatus Liberibacter solanacearum’ (Lso) can provide knowledge to better understand and combat zebra chip. This dissertation aims to understand the effect of biotic factors (e.g. insect and pathogen) and abiotic factor (e.g. water stress) affecting plant responses. First, the effects of jasmonate and salicylate defenses were tested on plant induced resistance against B. cockerelli. The induction of jasmonate defenses did not reduce B. cockerelli eggs and nymphs on the plants. However, the induction of salicylate defenses reduced eggs, nymphs and insect survival on plants treated with 1 mM of acetylsalicylic acid. Second, the effect of dehydration stress on plant susceptibility to B. cockerelli with Lso was studied to evaluate the effect of drought on the outbreak of B. cockerelli and Lso. Results from this study indicated that dehydration stress enhanced B. cockerelli survival and rendered the host plant more susceptible to B. cockerelli infestation. Finally, plant transcriptomes were studied to evaluate significantly differentially expressed genes (DEGs) in plant responses to three stages of B. cockerelli colonization and Lso infection. The comparison between plants exposed to no insects and plants exposed to Lso-negative B. cockerelli resulted in 207 DEGs. The comparison between plants exposed to no insects and plants exposed to Lso-positive B. cockerelli resulted in 404 DEGs. The comparison between plants exposed to Lso-negative B. cockerelli and plants exposed to Lso-positive B. cockerelli resulted in 471 DEGs. Plants exposed to Lso-negative B. cockerelli up-regulated the expression of several defense genes after one, two and four weeks of exposure. Plants exposed to Lso-positive B. cockerelli resulted in a delay of up-regulating plant defense genes. Lso-positive plants down-regulated the expression of photosynthesis genes and up-regulated carbohydrate metabolic genes as early signs of disease development. Overall, this dissertation shows that plant induced responses to biotic and abiotic stress are temporally dynamic and that plant induced defenses could be effective against B. cockerelli but not necessarily effective against Lso infection.